Developmental anomalies of the brain, including mental retardation and malformations, occur in approximately 2% of all live born children and epilepsy in about 0.5%, placing them among the most common known childhood disorders. Despite this high frequency, the molecular and cellular basis for only a very few disorders has been recently elucidated, while the basis of many more remains unknown. Mutations in the transcription factor ARX have been described in several children with early childhood epilepsy and mental retardation, both with and without associated brain malformations. As predicted in our first application, mutations of this gene prove to be a relatively common cause of mental retardation and infantile epilepsy based on the wide spectrum of severity already apparent in this group of children and a recurrent mechanism for mutation in at least two of the four polyalanine tracts found in the gene. The developmental mechanism by which ARX mutations result in this wide spectrum of problems is incompletely understood, although emerging data implicate disturbances in radial and nonradial cell migration, two pathways required for normal brain development. Based on data generated from this grant in humans and mice, the following hypotheses have been generated: (1) the type of ARX mutation predicts the phenotype in both hemizygous males and heterozygous females; (2) the phenotype in affected female humans and mice correlates with X-inactivation; and (3) Arx with an expanded poly-A tract results in defects in transcriptional repression, ultimate resulting in mice with seizures and mental retardation. To test these hypotheses, a series of experiments are proposed that will discover the mutation types in a large series of male and female patients with candidate phenotypes and determine X-inactivation status. In this application we will focus on elucidating the mechanism of a poly-A tract mutation in causing the neurologic phenotype, and in further understanding the downstream targets of Arx and their role in normal and abnormal brain development and function. The phenotypes in humans and mutant mice with different ARX/Arx mutations will be analyzed and compared to each other and to overlapping phenotypes caused by mutations of related genes. These studies are expected to provide a greater understanding of how Arx functions in normal and abnormal development, and will contribute to our understanding of the pathogenesis of such common disorders in children as mental retardation, epilepsy, and structural anomalies of the brain.